Image processing device, image processing method, and image processing program

ABSTRACT

A processor detects a structure of interest from a plurality of tomographic images indicating a plurality of tomographic planes of an object. The processor selects a tomographic image from the plurality of tomographic images according to a frequency band in a region in which the structure of interest has been detected. The processor generates a composite two-dimensional image using the selected tomographic image in the region in which the structure of interest has been detected and using a predetermined tomographic image in a region in which the structure of interest has not been detected.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation of PCT InternationalApplication No. PCT/JP2021/004848, filed on Feb. 9, 2021, which claimspriority to Japanese Patent Application No. 2020-047342, filed on Mar.18, 2020. Each application above is hereby expressly incorporated byreference, in its entirety, into the present application.

BACKGROUND Technical Field

The present disclosure relates to an image processing device, an imageprocessing method, and an image processing program.

Related Art

In recent years, image diagnosis using a radiography apparatus (calledmammography) for capturing an image of a breast has attracted attentionin order to promote early detection of breast cancer. Further, in themammography, tomosynthesis imaging has been proposed which moves aradiation source, irradiates the breast with radiation at a plurality ofradiation source positions to acquire a plurality of projection images,and reconstructs the plurality of acquired projection images to generatetomographic images in which desired tomographic planes have beenhighlighted. In the tomosynthesis imaging, the radiation source is movedin parallel to a radiation detector or is moved so as to draw a circularor elliptical arc according to the characteristics of an imagingapparatus and the required tomographic image, and imaging is performedon the breast at a plurality of radiation source positions to acquire aplurality of projection images. Then, the projection images arereconstructed using, for example, a back projection method, such as asimple back projection method or a filtered back projection method, or asequential reconstruction method to generate tomographic images.

The tomographic images are generated in a plurality of tomographicplanes of the breast, which makes it possible to separate structuresthat overlap each other in a depth direction in which the tomographicplanes are arranged in the breast. Therefore, it is possible to find anabnormal part such as a lesion that has been difficult to detect in atwo-dimensional image (hereinafter, referred to as a simpletwo-dimensional image) acquired by simple imaging according to therelated art which irradiates an object with radiation in a predetermineddirection.

In addition, a technique has been known which combines a plurality oftomographic images having different distances (positions in a heightdirection) from a detection surface of a radiation detector to aradiation source, which have been acquired by tomosynthesis imaging,using, for example, an addition method, an averaging method, a maximumintensity projection method, or a minimum intensity projection method togenerate a pseudo two-dimensional image (hereinafter, referred to as acomposite two-dimensional image) corresponding to the simpletwo-dimensional image (see JP2014-128716A).

In contrast, in the medical field, a computer aided diagnosis(hereinafter, referred to as CAD) system has been known whichautomatically detects a structure, such as an abnormal shadow, in animage and displays the detected structure so as to be highlighted. Forexample, the CAD is used to detect important diagnostic structures, suchas a tumor, a spicula, and a calcification, from the tomographic imagesacquired by the tomosynthesis imaging. In addition, a method has beenproposed which, in a case in which a composite two-dimensional image isgenerated from a plurality of tomographic images acquired by performingthe tomosynthesis imaging on the breast, detects a region of interestincluding a structure using the CAD and combines the detected region ofinterest on, for example, a projection image or a two-dimensional imageacquired by simple imaging to generate a composite two-dimensional image(see the specification of U.S. Pat. No. 8,983,156B). Further, a methodhas been proposed which averages and combines tomographic imagesincluding only the structure detected by the CAD to generate a compositetwo-dimensional image (see the specification of U.S. Pat. No.9,792,703B).

However, in the composite two-dimensional image generated by the methoddisclosed in the specification of U.S. Pt. No. 8,983,156B, the structureof interest combined with the two-dimensional image is only thestructure of interest acquired from one tomographic image. Therefore, ina case in which the structure of interest is present across a pluralityof tomographic images, it is not possible to reflect a state in whichthe structure of interest is present in a depth direction in which thetomographic images are arranged in the composite two-dimensional image.In addition, the method disclosed in the specification of U.S. Pat. No.9,792,703B averages the structures of interest included in a pluralityof tomographic images. Therefore, for example, a fine structure ofinterest, such as a calcification, and a linear structure, such asspicula, included in the breast are faint and difficult to see.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances,and an object of the present invention is to make it easy to see astructure of interest in a depth direction and a fine structure ofinterest included in an object in a composite two-dimensional image.

An image processing device according to the present disclosure comprisesat least one processor. The processor is configured to detect astructure of interest from a plurality of tomographic images indicatinga plurality of tomographic planes of an object, to select a tomographicimage from a plurality of tomographic images according to a frequencyband in a region in which a structure of interest has been detected andto generate a composite two-dimensional image using a selectedtomographic image in the region in which the structure of interest hasbeen detected and using a predetermined tomographic image in a region inwhich the structure of interest has not been detected.

In addition, in the image processing device according to the presentdisclosure, the processor may be configured to perform frequencydecomposition on the plurality of tomographic images to derive aplurality of band tomographic images for each of a plurality offrequency bands, to select a band tomographic image corresponding to thetomographic image in which the structure of interest has been detectedfor each pixel, which corresponds to a pixel of the compositetwo-dimensional image, in the plurality of band tomographic imagesaccording to the frequency band, and to generate the compositetwo-dimensional image using the selected band tomographic image in theregion in which the structure of interest has been detected.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to select different numbersof band tomographic images corresponding to the tomographic images inwhich the structure of interest has been detected from the plurality ofband tomographic images according to the frequency band. Furthermore,the different numbers may be 0. That is, the band tomographic image maynot be selected in a certain frequency band.

Moreover, in the image processing device according to the presentdisclosure, the plurality of frequency bands may include a firstfrequency band and a second frequency band lower than the firstfrequency band, and the processor may be configured to select a smallernumber of band tomographic images in the first frequency band than thatin the second frequency band.

In addition, in the image processing device according to the presentdisclosure, the processor may be configured to select all of the bandtomographic images including the structure of interest for each pixel,which corresponds to a pixel of the composite two-dimensional image, inthe plurality of band tomographic images in the second frequency band.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to select one bandtomographic image that best represents the structure of interest foreach pixel, which corresponds to a pixel of the compositetwo-dimensional image, in the plurality of band tomographic images inthe second frequency band.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to select one bandtomographic image that best represents the structure of interest foreach pixel position, which corresponds to a pixel position of thecomposite two-dimensional image, in the plurality of band tomographicimages in the first frequency band.

Furthermore, in the image processing device according to the presentdisclosure, the one band tomographic image that best represents thestructure of interest may be a band tomographic image having a largeststructure of interest or a band tomographic image having a highestlikelihood in a case in which the structure of interest is detected.

Moreover, in the image processing device according to the presentdisclosure, the processor may further select the band tomographic imageaccording to a type of the structure of interest.

In addition, in the image processing device according to the presentdisclosure, the structure of interest may be a tumor, a spicula, and acalcification.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to generate a composite bandtwo-dimensional image for each frequency band using the selected bandtomographic image in a pixel of the band tomographic image correspondingto the structure of interest and to perform frequency synthesis on thecomposite band two-dimensional images to generate the compositetwo-dimensional image.

Moreover, in the image processing device according to the presentdisclosure, the processor may be configured to generate the compositeband two-dimensional image that has a pixel value of a band tomographicimage determined on the basis of a predetermined priority of thestructure of interest in a case in which a plurality of the bandtomographic images are selected in pixels, which correspond to a pixelof the composite band two-dimensional image, in the plurality of bandtomographic images.

In addition, in the image processing device according to the presentdisclosure, the processor may be configured to combine the plurality oftomographic images to generate a first composite two-dimensional image,to generate a composite band two-dimensional image for each frequencyband using the band tomographic image selected for each type of thestructure of interest in a pixel of the band tomographic imagecorresponding to the structure of interest, to perform frequencysynthesis on the composite band two-dimensional images to generate asecond composite two-dimensional image for each type of the structure ofinterest, and to combine the second composite two-dimensional imagegenerated for each type of the structure of interest with the firstcomposite two-dimensional image to generate the compositetwo-dimensional image.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to replace a pixel value ofthe structure of interest in the first composite two-dimensional imagewith a pixel value of the structure of interest in the second compositetwo-dimensional image to combine the second composite two-dimensionalimage with the first composite two-dimensional image.

Furthermore, in the image processing device according to the presentdisclosure, the processor may be configured to generate the compositetwo-dimensional image having a pixel value of the second compositetwo-dimensional image determined on the basis of a predeterminedpriority of the structure of interest in a case in which a plurality oftypes of the structures of interest are included in corresponding pixelsof the plurality of second composite two-dimensional images.

In addition, in the image processing device according to the presentdisclosure, the processor may be configured to combine the plurality oftomographic images to generate a first composite two-dimensional image,to extract a region of a predetermined specific type of structure ofinterest from the first composite two-dimensional image, to generate acomposite band two-dimensional image for each frequency band, using theband tomographic image selected for each of types of structures ofinterest other than the specific type of structure of interest, inpixels of the band tomographic image which correspond to the otherstructures of interest, to perform frequency synthesis on the compositeband two-dimensional images to generate a second compositetwo-dimensional image for each type of the other structures of interest,to combine the second composite two-dimensional images for the otherstructures of interest with the first composite two-dimensional image,and to combine the region of the specific type of structure of interestwith the first composite two-dimensional image, with which the secondcomposite two-dimensional images have been combined, to generate thecomposite two-dimensional image.

Further, in the image processing device according to the presentdisclosure, the specific structure of interest may be a calcification,and the other structures of interest may be a tumor and a spicula.

Further, in the image processing device according to the presentdisclosure, the processor may be configured to replace a pixel value ofthe structure of interest in the first composite two-dimensional imagewith a pixel value of the structure of interest in the second compositetwo-dimensional image to combine the second composite two-dimensionalimage with the first composite two-dimensional image.

Furthermore, in the image processing device according to the presentdisclosure, the processor may be configured to generate the compositetwo-dimensional image having a pixel value of the second compositetwo-dimensional image determined on the basis of a predeterminedpriority of the structure of interest in a case in which a plurality oftypes of the other structures of interest are included in correspondingpixels of the plurality of second composite two-dimensional images.

Moreover, in the image processing device according to the presentdisclosure, the processor may be configured to replace a pixel value ofthe structure of interest in the first composite two-dimensional image,with which the second composite two-dimensional image has been combined,with a pixel value of the region of the specific type of structure ofinterest to combine the region of the specific type of structure ofinterest with the first composite two-dimensional image with which thesecond composite two-dimensional image has been combined.

An image processing method according to the present disclosure comprise:detecting a structure of interest from a plurality of tomographic imagesindicating a plurality of tomographic planes of an object; selecting atomographic image from the plurality of tomographic images according toa frequency band in a region in which the structure of interest has beendetected; and generating a composite two-dimensional image using theselected tomographic image in the region in which the structure ofinterest has been detected and using a predetermined tomographic imagein a region in which the structure of interest has not been detected.

In addition, a program that causes a computer to perform the imageprocessing method according to the present disclosure may be provided.

According to the present disclosure, it is possible to easily see astructure of interest in a depth direction and a fine structure ofinterest included in an object in a composite two-dimensional image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram schematically illustrating a configuration of aradiography system to which an image processing device according to anembodiment of the present disclosure is applied.

FIG. 2 is a diagram illustrating a radiography apparatus as viewed froma direction of an arrow A in FIG. 1 .

FIG. 3 is a diagram schematically illustrating a configuration of theimage processing device according to a first embodiment.

FIG. 4 is a diagram illustrating a functional configuration of the imageprocessing device according to the first embodiment.

FIG. 5 is a diagram illustrating the acquisition of projection images.

FIG. 6 is a diagram illustrating the generation of tomographic images.

FIG. 7 is a diagram illustrating the detection of a structure ofinterest.

FIG. 8 is a diagram illustrating a detection result of the structure ofinterest.

FIG. 9 is a diagram illustrating band tomographic images.

FIG. 10 is a diagram illustrating the selection of a band tomographicimage for a tumor in a medium-low frequency band.

FIG. 11 is a diagram illustrating the selection of the band tomographicimage for the tumor in a high frequency band.

FIG. 12 is a diagram illustrating the selection of a band tomographicimage for a spicula in the high frequency band.

FIG. 13 is a diagram illustrating the selection of a band tomographicimage for a calcification in the high frequency band.

FIG. 14 is a diagram illustrating a spicula extending across a pluralityof tomographic images.

FIG. 15 is a diagram illustrating the generation of a composite bandtwo-dimensional image.

FIG. 16 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the medium-low frequency band.

FIG. 17 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the high frequency band.

FIG. 18 is a diagram illustrating a composite two-dimensional imagedisplay screen.

FIG. 19 is a flowchart illustrating a process performed in the firstembodiment.

FIG. 20 is a diagram illustrating the generation of a second compositeband two-dimensional image in the medium-low frequency band for thetumor.

FIG. 21 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the tumor.

FIG. 22 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the spicula.

FIG. 23 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for thecalcification.

FIG. 24 is a diagram illustrating the generation of a compositetwo-dimensional image CG0 in a second embodiment.

FIG. 25 is a flowchart illustrating a process performed in the secondembodiment.

FIG. 26 is a diagram illustrating the extraction of a calcificationregion.

FIG. 27 is a diagram illustrating the generation of the compositetwo-dimensional image in a third embodiment.

FIG. 28 is a flowchart illustrating a process performed in the thirdembodiment.

FIG. 29 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the high frequency band in a fourth embodiment.

FIG. 30 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the tumor inthe fourth embodiment.

FIG. 31 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the high frequency band in a fifth embodiment.

FIG. 32 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the tumor inthe fifth embodiment.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings. FIG. 1 is a diagram schematicallyillustrating a configuration of a radiography system to which an imageprocessing device according to an embodiment of the present disclosureis applied, and FIG. 2 is a diagram illustrating a mammography apparatusin the radiography system as viewed from a direction of an arrow A inFIG. 1 . As illustrated in FIG. 1 , a radiography system 100 accordingto this embodiment images a breast M, which is an object, at a pluralityof radiation source positions and acquires a plurality of radiographicimages, that is, a plurality of projection images, in order to performtomosynthesis imaging on the breast to generate tomographic images. Theradiography system 100 according to this embodiment comprises amammography apparatus 1, a console 2, an image storage system 3, and animage processing device 4.

The mammography apparatus 1 comprises an arm portion 12 that isconnected to a base (not illustrated) by a rotation shaft 11. An imagingtable 13 is attached to one end of the arm portion 12, and a radiationemitting unit 14 is attached to the other end of the arm portion 12 soas to face the imaging table 13. The arm portion 12 is configured suchthat only the end to which the radiation emitting unit 14 is attachedcan be rotated. Therefore, the imaging table 13 is fixed, and only theradiation emitting unit 14 can be rotated.

A radiation detector 15, such as a flat panel detector, is provided inthe imaging table 13. The radiation detector 15 has a detection surface15A for radiation. In addition, for example, a circuit substrateincluding a charge amplifier that converts a charge signal read from theradiation detector 15 into a voltage signal, a correlated doublesampling circuit that samples the voltage signal output from the chargeamplifier, and an analog-digital (AD) conversion unit that converts thevoltage signal into a digital signal is provided in the imaging table13.

A radiation source 16 is accommodated in the radiation emitting unit 14.The radiation source 16 emits, for example, X-rays as the radiation. Theconsole 2 controls the timing when the radiation source 16 emits theradiation and the radiation generation conditions of the radiationsource 16, that is, the selection of target and filter materials, a tubevoltage, an irradiation time, and the like.

Further, the arm portion 12 is provided with a compression plate 17 thatis disposed above the imaging table 13 and presses and compresses thebreast M, a support portion 18 that supports the compression plate 17,and a movement mechanism 19 that moves the support portion 18 in anup-down direction in FIGS. 1 and 2 . In addition, an interval betweenthe compression plate 17 and the imaging table 13, that is, acompression thickness is input to the console 2.

The console 2 has a function of controlling the mammography apparatus 1using, for example, an imaging order and various kinds of informationacquired from a radiology information system (RIS) (not illustrated) orthe like through a network, such as a wireless communication local areanetwork (LAN), and instructions or the like directly issued by aradiology technician or the like. Specifically, the console 2 directsthe mammography apparatus 1 to perform the tomosynthesis imaging on thebreast M, acquires a plurality of projection images as described below,and reconstructs the plurality of projection images to generate aplurality of tomographic images. For example, in this embodiment, aserver computer is used as the console 2.

The image storage system 3 is a system that stores image data such asradiographic images and tomographic images captured by the mammographyapparatus 1. The image storage system 3 extracts an image correspondingto a request from, for example, the console 2 and the image processingdevice 4 from the stored images and transmits the image to a device thatis the source of the request. A specific example of the image storagesystem 3 is a picture archiving and communication system (PACS).

Next, an image processing device according to a first embodiment will bedescribed. Next, a hardware configuration of the image processing deviceaccording to the first embodiment will be described with reference toFIG. 3 . As illustrated in FIG. 3 , the image processing device 4 is acomputer, such as a workstation, a server computer, or a personalcomputer, and comprises a central processing unit (CPU) 21, anon-volatile storage 23, and a memory 26 as a temporary storage area. Inaddition, the image processing device 4 comprises a display 24, such asa liquid crystal display, an input device 25, such as a keyboard and amouse, and a network interface (I/F) 27 that is connected to a network(not illustrated). The CPU 21, the storage 23, the display 24, the inputdevice 25, the memory 26, and the network I/F 27 are connected to a bus28. In addition, the CPU 21 is an example of a processor according tothe present disclosure.

The storage 23 is implemented by, for example, a hard disk drive (HDD),a solid state drive (SSD), and a flash memory. An image processingprogram 22 installed in the image processing device 4 is stored in thestorage 23 as a storage medium. The CPU 21 reads out the imageprocessing program 22 from the storage 23, expands the image processingprogram 22 in the memory 26, and executes the expanded image processingprogram 22.

In addition, the image processing program 22 is stored in a storagedevice of a server computer connected to the network or a networkstorage in a state in which it can be accessed from the outside and isdownloaded and installed in the computer constituting the imageprocessing device 4 as required. Alternatively, the programs arerecorded on a recording medium, such as a digital versatile disc (DVD)or a compact disc read only memory (CD-ROM), are distributed, and areinstalled in the computer constituting the image processing device 4from the recording medium.

Next, a functional configuration of the image processing deviceaccording to the first embodiment will be described. FIG. 4 is a diagramillustrating the functional configuration of the image processing deviceaccording to the first embodiment. As illustrated in FIG. 4 , the imageprocessing device 4 comprises an image acquisition unit 30, astructure-of-interest detection unit 31, a frequency decomposition unit32, a selection unit 33, a combination unit 34, and a display controlunit 35. Then, the CPU 21 executes the image processing program 22 tofunction as the image acquisition unit 30, the structure-of-interestdetection unit 31, the frequency decomposition unit 32, the selectionunit 33, the combination unit 34, and the display control unit 35.

The image acquisition unit 30 acquires the tomographic image acquired bythe imaging performed by the mammography apparatus 1 under the controlof the console 2. The image acquisition unit 30 acquires the tomographicimage from the console 2 or the image storage system 3 through thenetwork I/F 27.

Here, the tomosynthesis imaging and the generation of tomographic imagesin the console 2 will be described. In a case in which the tomosynthesisimaging for generating tomographic images is performed, the console 2rotates the arm portion 12 about the rotation shaft 11 to move theradiation source 16, irradiates the breast M, which is an object, withradiation at a plurality of radiation source positions caused by themovement of the radiation source 16 under predetermined imagingconditions for tomosynthesis imaging, detects the radiation transmittedthrough the breast M using the radiation detector 15, and acquires aplurality of projection images Gi (i=1 to n, where n is the number ofradiation source positions and is, for example, 15) at the plurality ofradiation source positions.

FIG. 5 is a diagram illustrating the acquisition of the projectionimages Gi. As illustrated in FIG. 5 , the radiation source 16 is movedto each of radiation source positions S1, S2, . . . , and Sn. Theradiation source 16 is driven at each radiation source position toirradiate the breast M with radiation. The radiation detector 15 detectsthe radiation transmitted through the breast M to acquire projectionimages G1, G2, . . . , and Gn corresponding to the radiation sourcepositions S1 to Sn, respectively. In addition, at each of the radiationsource positions S1 to Sn, the breast M is irradiated with the same doseof radiation.

Furthermore, in FIG. 5 , a radiation source position Sc is a radiationsource position where an optical axis X0 of the radiation emitted fromthe radiation source 16 is orthogonal to the detection surface 15A ofthe radiation detector 15. It is assumed that the radiation sourceposition Sc is referred to as a reference radiation source position Sc.

Then, the console 2 reconstructs the plurality of projection images Gito generate tomographic images in which the desired tomographic planesof the breast M have been highlighted. Specifically, the console 2reconstructs the plurality of projection images Gi using a known backprojection method, such as a simple back projection method or a filteredback projection method, to generate a plurality of tomographic images Dj(j=1 to m) in each of a plurality of tomographic planes of the breast Mas illustrated in FIG. 6 . In this case, a three-dimensional coordinateposition in a three-dimensional space including the breast M is set, thepixel values of the corresponding pixels in the plurality of projectionimages Gi are reconstructed for the set three-dimensional coordinateposition, and pixel values at the coordinate positions of the pixels arecalculated.

The console 2 directly transmits the generated tomographic images Dj tothe image processing device 4 or transmits the generated tomographicimages Dj to the image storage system 3.

The structure-of-interest detection unit 31 detects a structure ofinterest from the plurality of tomographic images Dj. In thisembodiment, a tumor, a spicula, and a calcification included in thebreast M are detected as the structures of interest. FIG. 7 is a diagramillustrating the detection of the structures of interest. Here, thedetection of the structures of interest from six tomographic images D1to D6 will be described. As illustrated in FIG. 7 , the tomographicimage D1 includes a calcification K13. The tomographic image D2 includesa tumor K21. The tomographic image D3 includes a tumor K31 which iscontiguous with the tumor K21 in the tomographic image D2 in the breastM and a spicula K32. The tomographic image D4 includes a tumor K41 awhich is contiguous with the tumor K21 in the tomographic image D2 andthe tumor K31 in the tomographic image D3 in the breast M, a tumor K41 bwhich is present only in the tomographic image D4, and a spicula K42.The tomographic image D5 includes a spicula K52. The tomographic imageD6 includes a calcification K63.

The structure-of-interest detection unit 31 detects the structure ofinterest from the tomographic images Dj using a known computer-aideddiagnosis (that is, CAD) algorithm. In the CAD algorithm, theprobability (likelihood) that the pixel in the tomographic images Djwill be the structure of interest is derived, and a pixel having aprobability equal to or greater than a predetermined threshold value isdetected as the structure of interest. In addition, the CAD algorithm isprepared for each type of structure of interest. In this embodiment, aCAD algorithm for detecting a tumor, a CAD algorithm for detecting aspicula, and a CAD algorithm for detecting a calcification are prepared.

Further, the detection of the structure of interest is not limited tothe method using the CAD. The structure of interest may be detected fromthe tomographic images Dj by a filtering process using a filter fordetecting the structure of interest, a detection model which has beensubjected to machine learning by deep learning and the like to detectthe structure of interest, and the like.

The structure-of-interest detection unit 31 detects the tumor, thespicula, and the calcification as the structures of interest from thetomographic images D1 to D6 illustrated in FIG. 7 and derives adetection result R1 of the tumor, a detection result R2 of the spicula,and a detection result R3 of the calcification as illustrated in FIG. 8. In the detection result R1 of the tumor, the tumor is detected in thetomographic images D2 to D4. In the detection result R2 of the spicula,the spicula is detected in the tomographic images D3 to D5. In thedetection result R3 of the calcification, the calcification is detectedin the tomographic images D1 and D6.

The frequency decomposition unit 32 performs frequency decomposition oneach of the plurality of tomographic images Dj to derive a plurality ofband tomographic images indicating frequency components in each of aplurality of frequency bands for each of the plurality of tomographicimages Dj. In addition, any known methods, such as wavelet transform andFourier transform, can be used as a frequency decomposition method, inaddition to a method for performing multiple resolution transformationon a radiographic image. Further, the number of bands obtained byfrequency decomposition may be two or more. Furthermore, in thisembodiment, a low frequency band, a medium frequency band, and a highfrequency band are described as the frequency bands. However, for thefrequency components included in the band tomographic images, the highfrequency band includes the largest number of frequency components,followed by the medium frequency band and the low frequency band in thisorder. Moreover, in a case in which the frequency is decomposed intofour or more frequency bands, the low frequency band, the mediumfrequency band, and the high frequency band can be set in any manner. Inaddition, in a case in which the number of bands obtained by thefrequency decomposition is two, it is assumed that the lower frequencyband is referred to as a medium-low frequency band and the higherfrequency band is referred to as a high frequency band. Further, even ina case in which the number of bands obtained by the frequencydecomposition is four or more, it is assumed that the low frequency bandand the medium frequency band may be collectively referred to as themedium-low frequency band.

FIG. 9 is a diagram illustrating band tomographic images. In addition,in FIG. 9 , for simplicity of description, only a medium-low frequencyband MLf and a high frequency band Hf are illustrated as a plurality offrequency bands. Further, the high frequency band Hf corresponds to afirst frequency band according to the present disclosure, and themedium-low frequency band MLf corresponds to a second frequency bandaccording to the present disclosure. Furthermore, it is assumed that theband tomographic images in the medium-low frequency band MLf arerepresented by DML1 to DML6 and the band tomographic images in the highfrequency band Hf are represented by DH1 to DH6. The band tomographicimages DML1 to DML6 in the medium-low frequency band MLf include onlythe tumor having a relatively large structure among the tumor, thespicula, and the calcification included in the tomographic images D1 toD6. The band tomographic images DH1 to DH6 in the high frequency bandinclude the spicula and the calcification having a fine structure andthe tumor having a fine structure.

In the region in which the structure of interest has been detected, theselection unit 33 selects a tomographic image from the plurality oftomographic images Dj according to the type of the structure of interestand the frequency band. Specifically, in the first embodiment, theselection unit 33 selects a band tomographic image corresponding to thetomographic image, in which the structure of interest has been detected,from the plurality of band tomographic images for each pixel, whichcorresponds to the pixels of a composite two-dimensional image CG0 whichwill be described below, in the plurality of band tomographic imagesaccording to the type of the structure of interest and the frequencyband. In addition, in a case in which a band tomographic image isselected, the selection unit 33 associates the position of the structureof interest in the tomographic images Dj with the positions of the bandtomographic images DMLj and DHj for each type of structure of interestdetected by the structure-of-interest detection unit 31.

FIG. 10 is a diagram illustrating the selection of the band tomographicimage for the tumor in the medium-low frequency band. FIG. 11 is adiagram illustrating the selection of the band tomographic image for thetumor in the high frequency band. FIG. 12 is a diagram illustrating theselection of the band tomographic image for the spicula in the highfrequency band. FIG. 13 is a diagram illustrating the selection of theband tomographic image for the calcification in the high frequency band.In addition, FIGS. 10 to 13 schematically illustrate the tomographicimages one-dimensionally. Further, in FIGS. 10 to 13 , an index 40 forshowing a correspondence relationship between the pixels of the bandtomographic image and the pixels of the composite two-dimensional imageCG0 is one-dimensionally illustrated. Furthermore, in the bandtomographic image including the structure of interest, the pixel of thedetected structure of interest is illustrated to be thicker than thepixels other than the structure of interest. Moreover, in FIGS. 10 and11 , the pixel of the tumor is painted black. In FIG. 12 , the pixel ofthe spicula is painted white. In FIG. 13 , the pixel of thecalcification is vertically hatched. In addition, in the index 40, 15pixels P1 to P15 corresponding to the pixels of the compositetwo-dimensional image CG0 are illustrated. Further, in the index 40,only the pixels P1, P5, P10, and P15 are denoted by reference numerals.Furthermore, in the following description, the same figures as FIGS. 10to 13 are illustrated in the same manner as FIGS. 10 to 13 .

First, the selection of the band tomographic image for the tumor will bedescribed. For the tumor, the selection unit 33 selects all of the bandtomographic images including the tumor for each pixel, which correspondsto the pixels of the composite two-dimensional image CG0, in a pluralityof band tomographic images in the medium-low frequency band MLf. Asillustrated in FIG. 10 , in the pixels P1, P4 to P6, and P11 to P15 ofthe band tomographic images DMLj in the medium-low frequency band MLf,no tumor is detected in all of the band tomographic images DMLj.Therefore, for the tumor, the selection unit 33 does not select any bandtomographic image for the pixels P1, P4 to P6, and P11 to P15. Further,in the pixels P2 and P3, the tumor is detected only in the bandtomographic image DML4. Therefore, the selection unit 33 selects theband tomographic image DML4 for the pixels P2 and P3. In the pixels P7and P10, the tumor is detected only in the band tomographic image DML3.Therefore, the selection unit 33 selects the band tomographic image DML3for the pixels P7 and P10. Furthermore, in the pixels P8 and P9, thetumor is detected in the band tomographic images DML2 to DML4.Therefore, the selection unit 33 selects all of the band tomographicimages DML2 to DML4 in which the tumor has been detected for the pixelsP8 and P9.

Meanwhile, in the high frequency band Hf, the selection unit 33 selectsone band tomographic image that best represents the tumor for eachpixel, which corresponds to the pixels of the composite two-dimensionalimage CG0, in the plurality of band tomographic images. As illustratedin FIG. 11 , in the pixels P1, P4 to P6, and P11 to P15 of the bandtomographic images DHj in the high frequency band Hf, no tumor isdetected in all of the band tomographic images DHj. Therefore, for thetumor, the selection unit 33 does not select any band tomographic imagefor the pixels P1, P4 to P6, and P11 to P15. Further, in the pixels P2and P3, the tumor is detected only in the band tomographic image DH4.Therefore, the selection unit 33 selects the band tomographic image DH4for the pixels P2 and P3. In the pixels P7 and P10, the tumor isdetected only in the band tomographic image DH3. Therefore, theselection unit 33 selects the band tomographic image DH3 for the pixelsP7 and P10. Furthermore, in the pixels P8 and P9, the tumor is detectedin the band tomographic images DH2 to DH4. Here, among the tumorsdetected in the band tomographic images DH2 to DH4, the tumor detectedin the band tomographic images DH3 is the largest, and the bandtomographic image DH3 among the band tomographic images DH2 to DH4 bestrepresents the tumor. Therefore, the selection unit 33 selects the bandtomographic image DH3 for the pixels P8 and P9. Moreover, instead of thelargest tumor, a band tomographic image including the tumor having thehighest probability (likelihood) derived by the structure-of-interestdetection unit 31 at the time of detection may be selected.

Next, the selection of the band tomographic image for the spicula willbe described. The structure of the spicula is included only in the bandtomographic image DHj in the high frequency band Hf. Therefore, theselection unit 33 selects one band tomographic image that bestrepresents the spicula for each pixel, which corresponds to the pixelsof the composite two-dimensional image CG0, in the plurality of bandtomographic images only in the high frequency band Hf As illustrated inFIG. 12 , in the pixels P1, P2, P7, and P12 to P15, no spicula isdetected in all of the band tomographic images DHj. Therefore, for thespicula, the selection unit 33 does not select any band tomographicimage DHj for the pixels P1, P2, P7, and P12 to P15. In addition, in thepixels P3, P4, and P11, the spicula is detected only in the bandtomographic image DH4. Therefore, the selection unit 33 selects the bandtomographic image DH4 for the pixels P3, P4, and P11. Further, in thepixels P5 and P10, the spicula is detected in the band tomographicimages DH4 and DHS. Here, of the spiculae detected in the bandtomographic images DH4 and DHS, the spicula detected in the bandtomographic images DH4 is the largest, and the band tomographic imageDH4 of the band tomographic images DH4 and DH5 best represents thespicula. Therefore, the selection unit 33 selects the band tomographicimage DH4 for the pixels P5 and P10. In addition, instead of the largestspicula, a tomographic image including the spicula having the highestprobability (likelihood) derived by the structure-of-interest detectionunit 31 at the time of detection may be selected.

In the pixels P6 and P9, the spicula is detected only in the bandtomographic image DH5. Therefore, the selection unit 33 selects the bandtomographic image DH5 for the pixels P6 and P9. Further, in the pixelP8, the spicula is detected in the band tomographic image DH3.Therefore, the selection unit 33 selects the band tomographic image DH3for the pixel P8.

Next, the selection of the band tomographic image for the calcificationwill be described. The structure of the calcification is included onlyin the band tomographic image DHj in the high frequency band Hf.Therefore, the selection unit 33 selects one band tomographic image thatbest represents the calcification for each pixel, which corresponds tothe pixels of the composite two-dimensional image CG0, in the pluralityof band tomographic images only in the high frequency band Hf. Asillustrated in FIG. 13 , in the pixels P1 to P11, P13, and P15, thecalcification is not detected in all of the band tomographic images DHj.Therefore, for the calcification, the selection unit 33 does not selectany band tomographic image for the pixels P1 to P11, P13, and P15. Inaddition, in the pixel P12, the calcification is detected only in theband tomographic image DH1. Therefore, the selection unit 33 selects theband tomographic image DH1 for the pixel P12. In the pixel P14, thecalcification is detected only in the band tomographic image DH6.Therefore, the selection unit 33 selects the band tomographic image DH6for the pixel P14.

Further, for the spicula, as illustrated in FIG. 14 , in some cases, onespicula K7 spreads two-dimensionally in a direction orthogonal to thetomographic plane and is present across a plurality of band tomographicimages DHk-1, DHk, and DHk+1. In this case, the spicula is detected in apixel P100 for the band tomographic image DHk−1, is detected in a pixelP101 for the band tomographic image DHk, and is detected in a pixel P102for the band tomographic image DHk+1. Therefore, in a case in which thespicula K7 is present in the breast M as illustrated in FIG. 14 and theband tomographic image DHk illustrated in FIG. 14 is selected, aplurality of band tomographic images DHk−1 and DHk+1, which are aboveand below the band tomographic image DHk and to which the spicula K7included in the band tomographic image DHk is connected, are alsoselected.

The combination unit 34 generates a composite two-dimensional imageusing the band tomographic images selected for each type of structure ofinterest by the selection unit 33 according to the frequency band.Specifically, the combination unit 34 generates a composite bandtwo-dimensional image for each frequency band using the selected bandtomographic images in the pixels of the band tomographic images, whichcorrespond to the structure of interest, and performs frequencysynthesis on the composite band two-dimensional images to generate acomposite two-dimensional image. The composite two-dimensional image isa pseudo two-dimensional image corresponding to a simple two-dimensionalimage that is captured by irradiating the breast M with radiationemitted at the reference radiation source position Sc. In thisembodiment, as illustrated in FIG. 15 , the combination unit 34generates a composite band two-dimensional image CGH0 by combining thepixel values of the corresponding pixels in each of the band tomographicimages DHj along a viewing direction from the reference radiation sourceposition Sc to the radiation detector 15, that is, along the opticalaxis X0 illustrated in FIG. 5 in a state in which the plurality of bandtomographic images (only DHj is illustrated in FIG. 15 ) are stacked.Hereinafter, the generation of the composite band two-dimensional imageor the composite two-dimensional image will be described.

FIG. 16 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the medium-low frequency band MLf. Asillustrated in FIG. 16 , for the pixels P1, P4 to P6, and P11 to P15 inwhich no tumor is detected, the combination unit 34 derives the addedaverage value of the pixel values of all of the band tomographic imagesDML1 to DML6 and sets the added average value as the pixel values of thepixels P1, P4 to P6, and P11 to P15 of the composite bandtwo-dimensional image CGML0 in the medium-low frequency band MLf. Inthis case, all of the band tomographic images are predeterminedtomographic images according to the present disclosure. Since the bandtomographic image DML4 in which the tumor has been detected is selectedfor the pixels P2 and P3, the combination unit 34 sets the pixel valuesof the pixels P2 and P3 of the band tomographic image DML4 as the pixelvalues of the pixels P2 and P3 of the composite band two-dimensionalimage CGML0. Since the band tomographic image DML3 is selected for thepixels P7 and P10 of the band tomographic image DMLj, the combinationunit 34 sets the pixel values of the pixels P7 and P10 of the bandtomographic image DML3 as the pixel values of the pixels P7 and P10 ofthe composite band two-dimensional image CGML0. Since the bandtomographic images DML2 to DML4 are selected for the pixels P8 and P9 ofthe band tomographic images DMLj, the combination unit 34 sets the addedvalue of the pixel values of the pixels P8 and P9 of the bandtomographic images DML2 to DML4 as the pixel values of the pixels P8 andP9 of the composite band two-dimensional image CGML0. In addition, aweighted added value, a weighted average value, or the like may be usedinstead of the added value. In this case, a weight for the bandtomographic image DML3 may be larger than those for the band tomographicimages DML2 and DML4.

FIG. 17 is a diagram illustrating the generation of the composite bandtwo-dimensional image in the high frequency band Hf. In addition, theband tomographic images DH3 and DH4 include both the tumor and thespicula. Therefore, in FIG. 17 , band tomographic images DH3-1 and DH4-1including the detection result of only the tumor and band tomographicimages DH3-2 and DH4-2 including the detection result of only thespicula are virtually illustrated side by side. Further, in thefollowing description, the same figures as FIG. 17 are illustrated inthe same manner as FIG. 17 .

As illustrated in FIG. 17 , for the pixels P1, P13, and P15 in whichnone of the structures of interest of the tumor, the spicula, and thecalcification are detected, the combination unit 34 derives the addedaverage value of the pixel values of all of the band tomographic imagesDH1 to DH6 and sets the added average value as the pixel values of thepixels P1, P13, and P15 of the composite band two-dimensional image CGH0in the high frequency band Hf. Since the band tomographic image DH4 inwhich the tumor has been detected is selected for the pixel P2, thecombination unit 34 sets the pixel value of the pixel P2 of the bandtomographic image DH4 as the pixel value of the pixel P2 of thecomposite band two-dimensional image CGH0. Since the band tomographicimage DH4 in which the tumor and the spicula have been detected isselected for the pixel P3, the combination unit 34 sets the pixel valueof the pixel P3 of the band tomographic image DH4 as the pixel value ofthe pixel P3 of the composite band two-dimensional image CGH0. Since theband tomographic image DH4 in which the spicula has been detected isselected for the pixels P4 and P5, the combination unit 34 sets thepixel values of the pixels P4 and P5 of the band tomographic image DH4as the pixel values of the pixels P4 and P5 of the composite bandtwo-dimensional image CGH0.

Since the band tomographic image DH5 in which the spicula has beendetected is selected for the pixel P6, the combination unit 34 sets thepixel value of the pixel P6 of the band tomographic image DH5 as thepixel value of the pixel P6 of the composite band two-dimensional imageCGH0. Since the band tomographic image DH3 in which the tumor has beendetected is selected for the pixel P7, the combination unit 34 sets thepixel value of the pixel P7 of the band tomographic image DH3 as thepixel value of the pixel P7 of the composite band two-dimensional imageCGH0. Since the band tomographic image DH3 in which the tumor and thespicula are detected is selected for the pixel P8, the combination unit34 sets the pixel value of the pixel P8 of the band tomographic imageDH3 as the pixel value of the pixel P8 of the composite bandtwo-dimensional image CGH0.

For the pixel P9, the band tomographic image DH3 in which the tumor hasbeen detected and the band tomographic image DH5 in which the spiculahas been detected are selected. In this embodiment, in a case in whichdifferent band tomographic images are selected for the tumor, thespicula, and the calcification in the same pixel of the band tomographicimages DHj, the pixel values of the band tomographic images determinedon the basis of priority given in the order of the tumor, the spicula,and the calcification are assigned. Therefore, the combination unit 34sets the pixel value of the pixel P9 of the band tomographic image DH5in which the spicula has been detected as the pixel value of the pixelP9 of the composite band two-dimensional image CGH0.

For the pixel P10, the band tomographic image DH3 in which the tumor hasbeen detected and the band tomographic image DH4 in which the spiculahas been detected are selected. Therefore, the combination unit 34 setsthe pixel value of the pixel P10 of the band tomographic image DH4 inwhich the spicula has been detected as the pixel value of the pixel P10of the composite band two-dimensional image CGH0.

Since the band tomographic image DH4 in which the spicula has beendetected is selected for the pixel P11, the combination unit 34 sets thepixel value of the pixel P11 of the band tomographic image DH4 as thepixel value of the pixel P11 of the composite band two-dimensional imageCGH0. Since the band tomographic image DH1 in which the calcificationhas been detected is selected for the pixel P12, the combination unit 34sets the pixel value of the pixel P12 of the band tomographic image DH1as the pixel value of the pixel P12 of the composite bandtwo-dimensional image CGH0. Since the band tomographic image DH6 inwhich the calcification has been detected is selected for the pixel P14,the combination unit 34 sets the pixel value of the pixel P14 of theband tomographic image DH6 as the pixel value of the pixel P14 of thecomposite band two-dimensional image CGH0.

Then, the combination unit 34 performs frequency synthesis on thecomposite band two-dimensional image CGML0 in the medium-low frequencyband MLf and the composite band two-dimensional image CGH0 in the highfrequency band Hf to generate a composite band two-dimensional image CGA method corresponding to the frequency decomposition performed by thefrequency decomposition unit 32 may be used as a frequency synthesismethod. For example, in a case in which the frequency decomposition isperformed by wavelet transform, the frequency synthesis may be performedby inverse wavelet transform.

The display control unit 35 displays the composite two-dimensional imageCG0 generated by the combination unit 34 on the display 24. FIG. 18 is adiagram illustrating a composite two-dimensional image display screen.As illustrated in FIG. 18 , the composite two-dimensional image CG0 isdisplayed on a display screen 50 of the display 24. In addition, thecomposite two-dimensional image CG0 illustrated in FIG. 18 is generatedfrom the tomographic images D1 to D6 illustrated in FIG. 7 . Thecomposite two-dimensional image CG0 illustrated in FIG. 18 clearlyincludes the calcification K13 included in the tomographic image D1, thetumor K31 included in the tomographic image D3, the tumor K41 b includedin the tomographic image D4, the calcification K63 included in thetomographic image D6, and the spiculae K32, K42, and K52 included in thetomographic images D3 to D5. Further, the illustration of the spiculaeK32, K42, and K52 is omitted. The spiculae K32, K42, and K52 partiallyoverlap the tumor K31, and the pixel values of the tumor K31 arereplaced with the pixel values of the spiculae K42 and K52 included inthe tomographic images D4 and D5.

Next, a process performed in the first embodiment will be described.FIG. 19 is a flowchart illustrating the process performed in the firstembodiment. In addition, it is assumed that the plurality of tomographicimages Dj are acquired in advance and stored in the storage 23. Theprocess is started in a case in which the input device 25 receives aprocess start instruction from the operator, and thestructure-of-interest detection unit 31 detects the structure ofinterest from each of the plurality of tomographic images Dj (StepST11). Then, the frequency decomposition unit 32 performs frequencydecomposition on each of the plurality of tomographic images Dj toderive a plurality of band tomographic images indicating frequencycomponents in each of a plurality of frequency bands for each of theplurality of tomographic images Dj (Step ST12).

Then, the selection unit 33 selects a band tomographic imagecorresponding to the tomographic image, in which the structure ofinterest has been detected, for each corresponding pixel in theplurality of band tomographic images from the plurality of bandtomographic images according to the type of the structure of interestand the frequency band (Step ST13).

Then, the combination unit 34 generates the composite bandtwo-dimensional images CGML0 and CGH0 using the selected bandtomographic images (Step ST14) and performs frequency synthesis on thecomposite band two-dimensional images CGML0 and CGH0 to generate thecomposite two-dimensional image CG0 (Step ST15). Then, the displaycontrol unit 35 displays the composite two-dimensional image CG0 on thedisplay 24 (Step ST16). Then, the process ends.

As described above, in the first embodiment, frequency banddecomposition is performed on the tomographic image, and a bandtomographic image including the structure of interest is selected from aplurality of band tomographic images DMLj and DHj according to the typeof the structure of interest and the frequency band. Then, in the regionin which the structure of interest has been detected, the compositetwo-dimensional image CG0 is generated using the selected bandtomographic image. Therefore, the composite two-dimensional image CG0 isgenerated using a smaller number of tomographic images in the region ofthe structure of interest, as compared to a case in which the compositetwo-dimensional image is generated by weighting and averaging all of thetomographic images as in the method disclosed in U.S. Pat. No.9,792,703B. As a result, in the composite two-dimensional image CG0, afine structure of interest is not blurred. In particular, in the firstembodiment, one band tomographic image that best represents thestructure of interest is selected for each corresponding pixel in theplurality of band tomographic images. Therefore, it is possible toreduce the blurring of a fine structure of interest in the compositetwo-dimensional image CG0.

Further, in the first embodiment, in the medium-low frequency band MLf,all of the band tomographic images including the structure of interestare selected for each pixel, which corresponds to the pixels of thecomposite two-dimensional image CG0, in the plurality of bandtomographic images. Therefore, even in a case in which one structure ofinterest spreads in a direction in which the band tomographic images arearranged, that is, in the depth direction of the breast M, the compositetwo-dimensional image CG0 is generated using a plurality of selectedband tomographic images, which makes it possible to reflect the state ofthe structure of interest in the depth direction in the compositetwo-dimensional image CG0.

Furthermore, in the first embodiment, in the high frequency band Hf, oneband tomographic image that best represents the structure of interest isselected for each pixel, which corresponds to the pixels of thecomposite two-dimensional image CG0, in the plurality of bandtomographic images. Therefore, even in a case in which one structure ofinterest spreads in the direction in which the band tomographic imagesare arranged, that is, in the depth direction of the breast M whiletwo-dimensionally spreading in a direction orthogonal to the opticalaxis X0 of radiation, a plurality of band tomographic images areselected for the structure of interest. Therefore, the compositetwo-dimensional image CG0 is generated using a plurality of selectedband tomographic images, which makes it possible to reflect the state ofthe structure of interest, which spreads in the depth direction whilespreading two-dimensionally, in the composite two-dimensional image CG0.

Further, in a case in which different band tomographic images areselected for the tumor, the spicula, and the calcification in the samepixel of the band tomographic images DMLj and DHj, the pixel values ofthe band tomographic images determined on the basis of priority given inthe order of the tumor, the spicula, and the calcification are assigned.Here, for the breast M, the tumor has the highest degree of malignancy,followed by the spicula and the calcification in this order. Therefore,the selection of the band tomographic image based on the above-mentionedpriority makes it possible to generate the composite two-dimensionalimage CG0 such that the structure of interest having a higher degree ofmalignancy is more conspicuous.

Next, a second embodiment of the present disclosure will be described.In addition, the configuration of an image processing device accordingto the second embodiment is the same as the configuration of the imageprocessing device according to the first embodiment except only theprocess to be performed. Therefore, the detailed description of thedevice will not be repeated here. In the second embodiment, thecombination unit 34 combines a plurality of tomographic images Dj togenerate a first composite two-dimensional image CG1. Then, for each ofthe structures of interest, the combination unit 34 generates acomposite band two-dimensional image for each frequency band using theselected band tomographic image in the pixel of the band tomographicimage corresponding to the structure of interest and performs frequencysynthesis on the composite band two-dimensional images to generatesecond composite two-dimensional images CG21, CG22, and CG23 for each ofthe structures of interest. Further, the combination unit 34 combinesthe second composite two-dimensional images CG21, CG22, and CG23 foreach of the structures of interest with the first compositetwo-dimensional image CG1 to generate a composite two-dimensional imageCG0.

In the second embodiment, first, the combination unit 34 combines theplurality of tomographic images Dj to generate the first compositetwo-dimensional image CG1. Specifically, the first compositetwo-dimensional image CG1 is generated by, for example, adding andaveraging the pixel values of the corresponding pixels in the pluralityof tomographic images Dj.

Further, in the second embodiment, the combination unit 34 generates thesecond composite two-dimensional images CG21, CG22, and CG23 accordingto the type of the structure of interest and the frequency band. Thatis, the second composite two-dimensional image CG21 for the tumor, thesecond composite two-dimensional image CG22 for the spicula, and thesecond composite two-dimensional image CG23 for the calcification aregenerated. First, the generation of the second composite two-dimensionalimage CG21 for the tumor will be described. In addition, the selectionunit 33 selects the band tomographic image for each frequency band foreach of the tumor, the spicula, and the calcification as in the firstembodiment.

In the second embodiment, the combination unit 34 generates a secondcomposite band two-dimensional image CGML21 using only the selected bandtomographic image only in the pixel in which the tumor has beendetected. First, the generation of the second composite bandtwo-dimensional image CGML21 in the medium-low frequency band MLf willbe described. In addition, for the tumor, band tomographic images DML2to DML4 are selected in the medium-low frequency band MLf. FIG. 20 is adiagram illustrating the generation of the second composite bandtwo-dimensional image in the medium-low frequency band for the tumor.

First, for the pixels P1, P4 to P6, and P11 to P15 in which no tumor isdetected in any of the band tomographic images DMLj, the combinationunit 34 derives the added average value of the pixel values of the bandtomographic images DML1 to DML6 and sets the added average value as thepixel values of the pixels P1, P4 to P6, and P11 to P15 of the secondcomposite band two-dimensional image CGML21 in the medium-low frequencyband MLf. Since the band tomographic image DML4 is selected for thepixels P2 and P3, the combination unit 34 sets the pixel values of thepixels P2 and P3 of the band tomographic image DML4 as the pixel valuesof the pixels P2 and P3 of the second composite band two-dimensionalimage CGML21. Since the band tomographic image DML3 is selected for thepixels P7 and P10 of the band tomographic images DMLj, the combinationunit 34 sets the pixel values of the pixels P7 and P10 of the bandtomographic image DML3 as the pixel values of the pixels P7 and P10 ofthe second composite band two-dimensional image CGML21. Since the bandtomographic images DML2 to DML4 are selected for the pixels P8 and P9 ofthe band tomographic images DMLj, the combination unit 34 sets the addedvalue of the pixel values of the pixels P8 and P9 of the bandtomographic images DML2 to DML4 as the pixel values of the pixels P8 andP9 of the second composite band two-dimensional image CGML21. Inaddition, a weighted added value, a weighted average value, or the likemay be used instead of the added value. In this case, a weight for theband tomographic image DML3 may be larger than those for the bandtomographic images DML2 and DML4.

Next, the generation of a second composite band two-dimensional imageCGH21 in the high frequency band Hf for the tumor will be described.FIG. 21 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the tumor. Asillustrated in FIG. 21 , for the pixels P1, P4 to P6, and P11 to P15 inwhich no tumor is detected in any of the band tomographic images DHj,the combination unit 34 derives the added average value of the pixelvalues of the band tomographic images DH1 to DH6 and sets the addedaverage value as the pixel values of the pixels P1, P4 to P6, and P11 toP15 of the second composite band two-dimensional image CGH21 in the highfrequency band Hf. Since the band tomographic image DH4 is selected forthe pixels P2 and P3, the combination unit 34 sets the pixel values ofthe pixels P2 and P3 of the band tomographic image DH4 as the pixelvalues of the pixels P2 and P3 of the second composite bandtwo-dimensional image CGH21. Since the band tomographic image DH3 isselected for the pixels P7 to P10 of the band tomographic images DHj,the combination unit 34 sets the pixel values of the pixels P7 to P10 ofthe band tomographic image DH3 as the pixel values of the pixels P7 toP10 of the second composite band two-dimensional image CGH21.

Then, the combination unit 34 performs frequency synthesis on the secondcomposite band two-dimensional image CGML21 in the medium-low frequencyband MLf and the second composite band two-dimensional image CGH2 in thehigh frequency band Hf for the tumor to generate the second compositetwo-dimensional image CG21 for the tumor.

Next, the generation of the second composite two-dimensional image CG22for the spicula will be described. In the second embodiment, also forthe spicula, the combination unit 34 generates the second composite bandtwo-dimensional image CG22 using only the selected band tomographicimage only in the pixel in which the spicula has been detected. Inaddition, the structure of the spicula is included only in the bandtomographic images DHj in the high frequency band Hf. Therefore, for theband tomographic images DMLj in the medium-low frequency band MLf, thecombination unit 34 sets the added average value of the pixel values ofall of the pixels P1 to P15 as the pixel values of the pixels P1 to P15of a second composite band two-dimensional image CGML22 in themedium-low frequency band MLf.

FIG. 22 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for the spicula.As illustrated in FIG. 22 , for the pixels P1, P2, P7, and P12 to P15 inwhich the spicula is not detected in any of the band tomographic imagesDHj, the combination unit 34 derives the added average value of thepixel values of the band tomographic images DH1 to DH6 and sets theadded average value as the pixel values of the pixels P1, P2, P7, andP12 to P15 of a second composite band two-dimensional image CGH22 in thehigh frequency band Hf Since the band tomographic image DH4 is selectedfor the pixels P3 to P5, P10, and P11, the combination unit 34 sets thepixel values of the pixels P3 to P5, P10, and P11 of the bandtomographic image DH4 as the pixel values of the pixels P3 to P5, P10,and P11 of the second composite band two-dimensional image CGH22. Sincethe band tomographic image DH5 is selected for the pixels P6 and P9, thecombination unit 34 sets the pixel values of the pixels P6 and P9 of theband tomographic image DH5 as the pixel values of the pixels P6 and P9of the second composite band two-dimensional image CGH22. Since the bandtomographic image DH3 is selected for the pixel P8, the combination unit34 uses the pixel value of the pixel P8 of the band tomographic imageDH3 as the pixel value of the pixel P8 of the second composite bandtwo-dimensional image CGH22.

Then, the combination unit 34 performs frequency synthesis on the secondcomposite band two-dimensional image CGML22 in the medium-low frequencyband MLf and the second composite band two-dimensional image CGH22 inthe high frequency band Hf for the spicula to generate the secondcomposite two-dimensional image CG22 for the spicula.

Next, the generation of the second composite two-dimensional image CG23for the calcification will be described. In the second embodiment, alsofor the calcification, the combination unit 34 generates the secondcomposite band two-dimensional image CG23 using only the selected bandtomographic image only in the pixel in which the calcification has beendetected. In addition, the structure of the calcification is includedonly in the band tomographic images DHj in the high frequency band HfTherefore, for the band tomographic images DMLj in the medium-lowfrequency band MLf, the combination unit 34 sets the added average valueof the pixel values of all of the pixels P1 to P15 as the pixel valuesof the pixels P1 to P15 of a second composite band two-dimensional imageCGML23 in the medium-low frequency band MLf.

FIG. 23 is a diagram illustrating the generation of the second compositeband two-dimensional image in the high frequency band for thecalcification. As illustrated in FIG. 23 , for the pixels P1 to P11,P13, and P15 in which the calcification is not detected in any of theband tomographic images DHj, the combination unit 34 derives the addedaverage value of the pixel values of the band tomographic images DH1 toDH6 and sets the added average value as the pixel values of the pixelsP1 to P11, P13, and P15 of a second composite band two-dimensional imageCGH23 in the high frequency band Hf Since the band tomographic image DH1is selected for the pixel P12, the combination unit 34 sets the pixelvalue of the pixel P12 of the band tomographic image DH1 in the highfrequency band Hf as the pixel value of the pixel P12 of the secondcomposite band two-dimensional image CGH23. Since the band tomographicimage DH6 is selected for the pixel P14, the combination unit 34 setsthe pixel value of the pixel P14 of the band tomographic image DH6 asthe pixel value of the pixel P14 of the second composite bandtwo-dimensional image CGH23.

Then, the combination unit 34 performs frequency synthesis on the secondcomposite band two-dimensional image CGML23 in the medium-low frequencyband MLf and the second composite band two-dimensional image CGH23 inthe high frequency band Hf for the calcification to generates the secondcomposite two-dimensional image CG23 for the calcification.

The combination unit 34 sequentially combines the second compositetwo-dimensional image CG21 for the tumor, the second compositetwo-dimensional image CG22 for the spicula, and the second compositetwo-dimensional image CG23 for the calcification generated as describedabove with the first composite two-dimensional image CG1 to generate thecomposite two-dimensional image CG0. FIG. 24 is a diagram illustratingthe generation of the composite two-dimensional image CG0 in the secondembodiment. As illustrated in FIG. 24 , first, the combination unit 34replaces the region of the tumor in the first composite two-dimensionalimage CG1 with the region of the tumor in the second compositetwo-dimensional image CG21 for the tumor to combine the second compositetwo-dimensional image CG21 for the tumor with the first compositetwo-dimensional image CG1. As a result, an intermediate compositetwo-dimensional image CG11 is generated.

Then, the combination unit 34 replaces the region of the spicula in theintermediate composite two-dimensional image CG11 with the region of thespicula in the second composite two-dimensional image CG22 for thespicula to combine the second composite two-dimensional image CG22 forthe spicula with the intermediate composite two-dimensional image CG11.As a result, an intermediate composite two-dimensional image CG12 isgenerated.

Further, the combination unit 34 replaces the region of thecalcification in the intermediate composite two-dimensional image CG12with the region of the calcification in the second compositetwo-dimensional image CG23 for the calcification to combine the secondcomposite two-dimensional image CG23 for the calcification with theintermediate composite two-dimensional image CG12. As a result, thecomposite two-dimensional image CG0 according to the second embodimentis generated.

Next, a process performed in the second embodiment will be described.FIG. 25 is a flowchart illustrating the process performed in the secondembodiment. In addition, it is assumed that the plurality of tomographicimages Dj are acquired in advance and stored in the storage 23. Theprocess is started in a case in which the input device 25 receives aprocess start instruction from the operator, and thestructure-of-interest detection unit 31 detects the structure ofinterest from the plurality of tomographic images Dj (Step ST21). Then,the frequency decomposition unit 32 performs frequency decomposition oneach of the plurality of tomographic images Dj to derive a plurality ofband tomographic images indicating frequency components in each of aplurality of frequency bands for each of the plurality of tomographicimages Dj (Step ST22).

Then, the selection unit 33 selects a band tomographic imagecorresponding to the tomographic image, in which the structure ofinterest has been detected, from the plurality of band tomographicimages for each corresponding pixel of the plurality of band tomographicimages according to the type of the structure of interest and thefrequency band (Step ST23).

Then, the combination unit 34 generates the first compositetwo-dimensional image CG1 from the plurality of tomographic images Dj(Step ST24). In addition, the process in Step ST24 may be performedbefore each of the processes in Steps ST21 to ST23 or may be performedin parallel to these processes. Then, the combination unit 34 generatesthe second composite two-dimensional images CG21, CG22, and CG23 for thetumor, the spicula, and the calcification, respectively (Step ST25).Further, the combination unit 34 sequentially combines the secondcomposite two-dimensional images CG21, CG22, and CG23 for the tumor, thespicula, and the calcification with the first composite two-dimensionalimage CG1 to generate the composite two-dimensional image CG0 (StepST26). Then, the display control unit 35 displays the compositetwo-dimensional image CG0 on the display 24 (Step ST27). Then, theprocess ends.

Next, a third embodiment of the present disclosure will be described. Inaddition, the configuration of an image processing device according tothe third embodiment is the same as the configuration of the imageprocessing device according to the second embodiment except only theprocess to be performed. Therefore, the detailed description of thedevice will not be repeated here. In the third embodiment, thecombination unit 34 combines the plurality of tomographic images Dj togenerate the first composite two-dimensional image CG1. Then, thecombination unit 34 generates a composite band two-dimensional image foreach frequency band using the selected band tomographic image in thepixels of the composite two-dimensional image CG0 which correspond tothe tumor and the spicula among the tumor, the spicula, and thecalcification and performs frequency synthesis on the composite bandtwo-dimensional images to generate the second composite two-dimensionalimages CG21 and CG22. Meanwhile, the combination unit 34 extracts theregion of the calcification as a calcification region from the firstcomposite two-dimensional image CG1. Further, the combination unit 34combines the second composite two-dimensional images CG21 and G22 forthe tumor and the spicula with the first composite two-dimensional imageCG1 and further combines the calcification region to generate thecomposite two-dimensional image CG0.

In addition, in the third embodiment, the generation of the firstcomposite two-dimensional image CG1, the generation of the secondcomposite band two-dimensional image CG21 for the tumor, and thegeneration of the second composite two-dimensional image CG22 for thespicula are performed by the combination unit 34 in the same manner asin the second embodiment.

FIG. 26 is a diagram illustrating the extraction of the calcificationregion from the first composite two-dimensional image CG1. Thecombination unit 34 extracts regions corresponding to the calcificationregions detected from each of the tomographic images Dj by thestructure-of-interest detection unit 31 as calcification regions 42A and42B from the first composite two-dimensional image CG1.

FIG. 27 is a diagram illustrating the generation of the compositetwo-dimensional image CG0 in the third embodiment. As illustrated inFIG. 27 , first, the combination unit 34 replaces the region of thetumor in the first composite two-dimensional image CG1 with the regionof the tumor in the second composite two-dimensional image CG21 for thetumor to combine the second composite two-dimensional image CG21 for thetumor with the first composite two-dimensional image CG1. As a result,an intermediate composite two-dimensional image CG11 is generated.

Then, the combination unit 34 replaces the region of the spicula in theintermediate composite two-dimensional image CG11 with the region of thespicula in the second composite two-dimensional image CG22 for thespicula to combine the second composite two-dimensional image CG22 forthe spicula with the intermediate composite two-dimensional image CG11.As a result, an intermediate composite two-dimensional image CG12 isgenerated.

Further, in the third embodiment, the combination unit 34 replaces thecalcification region of the intermediate composite two-dimensional imageCG12 with the calcification regions 42A and 42B to combine thecalcification regions 42A and 42B with the intermediate compositetwo-dimensional image CG12. As a result, the composite two-dimensionalimage CG0 according to the third embodiment is generated.

Next, a process performed in the third embodiment will be described.FIG. 28 is a flowchart illustrating the process performed in the thirdembodiment. In addition, it is assumed that the plurality of tomographicimages Dj are acquired in advance and stored in the storage 23. Theprocess is started in a case in which the input device 25 receives aprocess start instruction from the operator, and thestructure-of-interest detection unit 31 detects the structure ofinterest from each of the plurality of tomographic images Dj (StepST31). Then, the frequency decomposition unit 32 performs frequencydecomposition on each of the plurality of tomographic images Dj toderive a plurality of band tomographic images indicating frequencycomponents in each of a plurality of frequency bands for each of theplurality of tomographic images Dj (Step ST32).

Then, the selection unit 33 selects a band tomographic imagecorresponding to the tomographic image, in which the structure ofinterest has been detected, from the plurality of band tomographicimages for each corresponding pixel of the plurality of band tomographicimages according to the type of the structure of interest and thefrequency band (Step ST33).

Then, the combination unit 34 generates the first compositetwo-dimensional image CG1 from the plurality of tomographic images Dj(Step ST34). In addition, the process in Step ST34 may be performedbefore each of the processes in Steps ST31 to ST33 or may be performedin parallel to these processes. Then, the combination unit 34 generatesthe second composite two-dimensional images CG21 and CG22 for the tumorand the spicula, respectively (Step ST35). Further, the combination unit34 extracts the calcification regions 42A and 42B from the firstcomposite two-dimensional image CG1 (Step ST36). Furthermore, theprocess in Step ST36 may be performed before any process after the firstcomposite two-dimensional image CG1 is generated or may be performed inparallel to any process.

Then, the combination unit 34 sequentially combines the second compositetwo-dimensional images CG21 and CG22 for the tumor and the spicula withthe first composite two-dimensional image CG1 to generate theintermediate composite two-dimensional image CG12 (Step ST37). Then, thecombination unit 34 combines the calcification regions 42A and 42B withthe intermediate composite two-dimensional image CG12 to generate thecomposite two-dimensional image CG0 (Step ST38). Further, the displaycontrol unit 35 displays the composite two-dimensional image CG0 on thedisplay 24 (Step ST39). Then, the process ends.

In each of the above-described embodiments, for the tumor, in themedium-low frequency band MLf, all of the band tomographic imagesincluding the tumor are selected for each pixel, which corresponds tothe pixels of the composite two-dimensional image CG0, in the pluralityof band tomographic images. Further, in the high frequency band Hf, oneband tomographic image that best represents the tumor is selected foreach pixel, which corresponds to the pixels of the compositetwo-dimensional image CG0, in the plurality of band tomographic images.However, the selection of the band tomographic image is not limitedthereto. For the tumor, only in the medium-low frequency band MLf, allof the band tomographic images including the tumor may be selected foreach pixel, which corresponds to the pixels of the compositetwo-dimensional image CG0, in the plurality of band tomographic images.Hereinafter, this will be described as a fourth embodiment.

In a case in which the band tomographic image is selected as in thefourth embodiment and the process according to the first embodiment isperformed, the combination unit 34 generates the composite bandtwo-dimensional image CGML0 in the medium-low frequency band MLf as inthe first embodiment. On the other hand, in the fourth embodiment, theband tomographic image for the tumor is not selected in the highfrequency band Hf Therefore, the band tomographic images DH2 and DH3-1are not selected even for the pixels P2 and P7 illustrated in FIG. 17 .Therefore, in a case in which the process according to the firstembodiment is performed in the fourth embodiment, the combination unit34 derives the added average value of the pixel values of the pixels P2and P7 in all of the band tomographic images DH1 to DH6 and sets theadded average value as the pixel values of the pixels P2 and P7 of thecomposite band two-dimensional image CGH0 in the high frequency band Hfas illustrated in FIG. 29 , similarly to the pixels P1, P13, and P15.

Meanwhile, in a case in which the band tomographic image is selected inthe fourth embodiment and the process according to the second embodimentis performed, the combination unit 34 generates the composite bandtwo-dimensional image CGML21 in the medium-low frequency band MLf forthe tumor as in the second embodiment. On the other hand, in the fourthembodiment, the band tomographic image for the tumor is not selected inthe high frequency band Hf Therefore, the band tomographic images DH2and DH3-1 are not selected even for the pixels P2 and P7 illustrated inFIG. 21 . Therefore, in a case in which the process according to thesecond embodiment is performed in the fourth embodiment, the combinationunit 34 derives the added average value of the pixel values of thepixels P2 and P7 in all of the band tomographic images DH1 to DH6 andsets the added average value as the pixel values of the pixels P2 and P7of the second composite band two-dimensional image CGH21 in the highfrequency band Hf for the tumor as illustrated in FIG. 30 , similarly tothe pixels P1, P4 to P6, and P11 to P15.

Further, for the tumor, in both the high frequency band Hf and themedium-low frequency band MLf, one band tomographic image that bestrepresents the tumor may be selected for each pixel, which correspondsto the pixels of the composite two-dimensional image CG0, in theplurality of band tomographic images. Hereinafter, this will bedescribed as a fifth embodiment.

In the fifth embodiment, the selection unit 33 selects one bandtomographic image that best represents the tumor for each pixel whichcorresponds to the pixels of the composite two-dimensional image CG0 inthe medium-low frequency band MLf for the tumor. Specifically, theselection unit 33 selects the band tomographic image DML4 for the pixelP2 and P3 illustrated in FIG. 10 and selects the band tomographic imageDML3 for the pixels P7 to P10. Further, in the fifth embodiment, theband tomographic images DML2 and DML4 illustrated in FIG. 10 are notselected for the pixels P8 and P9. In the high frequency band Hf, theband tomographic image is selected in the same manner as in each of theabove-described embodiments.

In a case in which the band tomographic image is selected as in thefifth embodiment and the process according to the first embodiment isperformed, the combination unit 34 generates the composite bandtwo-dimensional image CGH0 in the high frequency band Hf as in the firstembodiment. Meanwhile, in the fifth embodiment, for the tumor, even inthe medium-low frequency band MLf, one band tomographic image that bestrepresents the tumor is selected for each pixel, which corresponds tothe pixels of the composite two-dimensional image CG0, in the pluralityof band tomographic images. Therefore, only one band tomographic imageDML3 is selected even for the pixels P8 and P9 illustrated in FIG. 16 .Therefore, in a case in which the process according to the firstembodiment is performed in the fifth embodiment, for the pixels P8 andP9, the combination unit 34 sets the pixel values of the pixels P8 andP9 of the band tomographic image DML3 as the pixel values of the pixelsP8 and P9 of the composite band two-dimensional image CGML0 in themedium-low frequency band MLf as illustrated in FIG. 31 .

Meanwhile, in a case in which the band tomographic image is selected asin the fifth embodiment and the process according to the secondembodiment is performed, the combination unit 34 generates the secondcomposite band two-dimensional image CGH21 in the high frequency band Hffor the tumor as in the second embodiment. Meanwhile, in the fifthembodiment, even in the medium-low frequency band MLf, one bandtomographic image that best represents the tumor is selected for eachpixel, which corresponds to the pixels of the composite two-dimensionalimage CG0, in the plurality of band tomographic images. Therefore, onlyone band tomographic image DML3 is selected even for the pixels P8 andP9 illustrated in FIG. 20 . Therefore, in a case in which the processaccording to the second embodiment is performed in the fifth embodiment,for the pixels P8 and P9, the combination unit 34 sets the pixel valuesof the pixels P8 and P9 of the band tomographic image DML3 as the pixelvalues of the pixels P8 and P9 of the composite band two-dimensionalimage CGML21 in the medium-low frequency band MLf for the tumor asillustrated in FIG. 32 .

Further, for the tumor, only in the medium-low frequency band MLf, oneband tomographic image that best represents the tumor may be selectedfor each pixel, which corresponds to the pixels of the compositetwo-dimensional image CG0, in the plurality of band tomographic images.Hereinafter, this will be described as a sixth embodiment.

In the sixth embodiment, the selection unit 33 selects one bandtomographic image that best represents the tumor for each pixel, whichcorresponds to the pixels of the composite two-dimensional image CG0,only in the medium-low frequency band MLf for the tumor. Specifically,the selection unit 33 selects the band tomographic image DML4 for thepixel P2 and P3 illustrated in FIG. 10 and selects the band tomographicimage DML3 for the pixels P7 to P10. Meanwhile, in the sixth embodiment,the band tomographic image is not selected in the high frequency band Hffor the tumor.

In a case in which the band tomographic image is selected as in thesixth embodiment and the process according to the first embodiment isperformed, the combination unit 34 generates the composite bandtwo-dimensional image CGH0 in the high frequency band Hf as in thefourth embodiment. Meanwhile, in the medium-low frequency band MLf, thecombination unit 34 generates the composite band two-dimensional imageCGML0 in the medium-low frequency band MLf as in the fifth embodiment.

Meanwhile, in a case in which the band tomographic image is selected asin the sixth embodiment and the process according to the secondembodiment is performed, the combination unit 34 generates the secondcomposite band two-dimensional image CGH21 in the high frequency band Hffor the tumor as in the fourth embodiment. Meanwhile, in the medium-lowfrequency band MLf, the combination unit 34 generates the composite bandtwo-dimensional image CGML21 in the medium-low frequency band MLf forthe tumor as in the fifth embodiment.

Further, in each of the above-described embodiments, for the pixels inwhich the structure of interest is not detected, in a case in which thecomposite band two-dimensional image is generated from the bandtomographic images, the added average value of the corresponding pixelsof the band tomographic images is set as the pixel values of thecomposite band two-dimensional image. However, the present disclosure isnot limited thereto. Furthermore, in the second and third embodiments,in a case in which the first composite two-dimensional image CG1 isgenerated, the added average value of the pixel values of thecorresponding pixels of the tomographic image Dj is used as the pixelvalues of the first composite two-dimensional image CG1. However, thepresent disclosure is not limited thereto. Further, in the second andthird embodiments, in a case in which the second composite bandtwo-dimensional image CGML22 in the medium-low frequency band MLf forthe spicula and the calcification is generated, the added average valueof the pixel values of the corresponding pixels of the band tomographicimages DMLj is set as the pixel values of the second composite bandtwo-dimensional image CGML22. However, the present disclosure is notlimited thereto. For example, other known techniques that use a weightedaverage value, a median value, or the like as the pixel value can beapplied. Further, a minimum intensity projection method using theminimum value of the corresponding pixels in each band tomographic imageor each tomographic image or a maximum intensity projection method usingthe maximum value may be used. In this case, a band tomographic image ora tomographic image including a pixel having the minimum value or themaximum value is a predetermined tomographic image according to thepresent disclosure.

In addition, for the pixels in which the structure of interest is notdetected, the average value of the corresponding pixels in each bandtomographic image or each tomographic image may be derived, a pixelhaving a value whose difference from the average value is smaller than apredetermined set value may be regarded as a noise pixel that is greatlyaffected by noise, and the pixel values of the composite bandtwo-dimensional image or the composite two-dimensional image may bederived excluding the noise pixel. Further, for the corresponding pixelsin each band tomographic image or each tomographic image, a variancevalue of pixel values in a predetermined region including the pixels maybe derived, a pixel having a variance value that is smaller than apredetermined set value may be regarded as a noise pixel, and the pixelvalues of the composite band two-dimensional image or the compositetwo-dimensional image may be derived excluding the noise pixel. In thiscase, a band tomographic image or a tomographic image having pixels thatare not the noise pixel is the predetermined tomographic image accordingto the present disclosure. Furthermore, a process that detects the edgeof a structure included in each band tomographic image or eachtomographic image may be performed. Then, for the pixels in which thestructure of interest is not detected, the pixel values of the pixelsincluding the edge may be used as the pixel values of the composite bandtwo-dimensional image or the composite two-dimensional image. In thiscase, a band tomographic image or a tomographic image having the pixelsincluding the edge is the predetermined tomographic image according tothe present disclosure.

Further, in each of the above-described embodiments, all of thestructures of interest of the tumor, the spicula, and the calcificationare detected. However, the present invention is not limited thereto. Thetechnology of the present disclosure can be applied even in a case inwhich at least one type of structure of interest among the tumor, thespicula, and the calcification is detected. In addition, in a case inwhich only one type of structure of interest is detected, the bandtomographic image may be selected according to only the frequency band.

Further, the radiation in each of the above-described embodiments is notparticularly limited. For example, a-rays or y-rays can be applied inaddition to the X-rays.

Furthermore, in each of the above-described embodiments, for example,the following various processors can be used as a hardware structure ofprocessing units performing various processes, such as the imageacquisition unit 30, the structure-of-interest detection unit 31, thefrequency decomposition unit 32, the selection unit 33, the combinationunit 34, and the display control unit 35. The various processorsinclude, for example, a CPU which is a general-purpose processorexecuting software (program) to function as various processing units asdescribed above, a programmable logic device (PLD), such as a fieldprogrammable gate array (FPGA), which is a processor whose circuitconfiguration can be changed after manufacture, and a dedicated electriccircuit, such as an application specific integrated circuit (ASIC),which is a processor having a dedicated circuit configuration designedto perform a specific process.

One processing unit may be configured by one of the various processorsor a combination of two or more processors of the same type or differenttypes (for example, a combination of a plurality of FPGAs or acombination of a CPU and an FPGA). In addition, a plurality ofprocessing units may be configured by one processor.

A first example of the configuration in which a plurality of processingunits are configured by one processor is an aspect in which oneprocessor is configured by a combination of one or more CPUs andsoftware and functions as a plurality of processing units. Arepresentative example of this aspect is a client computer or a servercomputer. A second example of the configuration is an aspect in which aprocessor that implements the functions of the entire system including aplurality of processing units using one integrated circuit (IC) chip isused. A representative example of this aspect is a system-on-chip (SoC).As such, various processing units are configured by using one or more ofthe various processors as a hardware structure.

In addition, specifically, an electric circuit (circuitry) obtained bycombining circuit elements, such as semiconductor elements, can be usedas the hardware structure of the various processors.

What is claimed is:
 1. An image processing device comprising at leastone processor, wherein the processor is configured to: detect astructure of interest from a plurality of tomographic images indicatinga plurality of tomographic planes of an object; select a tomographicimage from the plurality of tomographic images according to a frequencyband in a region in which the structure of interest has been detected;and generate a composite two-dimensional image using the selectedtomographic image in the region in which the structure of interest hasbeen detected and using a predetermined tomographic image in a region inwhich the structure of interest has not been detected.
 2. The imageprocessing device according to claim 1, wherein the processor isconfigured to perform frequency decomposition on the plurality oftomographic images to derive a plurality of band tomographic images foreach of a plurality of frequency bands, to select a band tomographicimage corresponding to the tomographic image in which the structure ofinterest has been detected for each pixel, which corresponds to a pixelof the composite two-dimensional image, in the plurality of bandtomographic images according to the frequency band; and generate thecomposite two-dimensional image using the selected band tomographicimage in the region in which the structure of interest has beendetected.
 3. The image processing device according to claim 2, whereinthe processor is configured to select different numbers of bandtomographic images corresponding to the tomographic images in which thestructure of interest has been detected from the plurality of bandtomographic images according to the frequency band.
 4. The imageprocessing device according to claim 2, wherein the plurality offrequency bands include a first frequency band and a second frequencyband lower than the first frequency band, and the processor isconfigured to select a smaller number of band tomographic images in thefirst frequency band than that in the second frequency band.
 5. Theimage processing device according to claim 4, wherein the processor isconfigured to select all of the band tomographic images including thestructure of interest for each pixel, which corresponds to a pixel ofthe composite two-dimensional image, in the plurality of bandtomographic images in the second frequency band.
 6. The image processingdevice according to claim 4, wherein the processor is configured toselect one band tomographic image that best represents the structure ofinterest for each pixel, which corresponds to a pixel of the compositetwo-dimensional image, in the plurality of band tomographic images inthe second frequency band.
 7. The image processing device according toclaim 4, wherein the processor is configured to select one bandtomographic image that best represents the structure of interest foreach pixel position, which corresponds to a pixel position of thecomposite two-dimensional image, in the plurality of band tomographicimages in the first frequency band.
 8. The image processing deviceaccording to claim 6, wherein the one band tomographic image that bestrepresents the structure of interest is a band tomographic image havinga largest structure of interest or a band tomographic image having ahighest likelihood in a case in which the structure of interest isdetected.
 9. The image processing device according to claim 2, whereinthe processor further selects the band tomographic image according to atype of the structure of interest.
 10. The image processing deviceaccording to claim 9, wherein the structure of interest is a tumor, aspicula, and a calcification.
 11. The image processing device accordingto claim 2, wherein the processor is configured to generate a compositeband two-dimensional image for each frequency band using the selectedband tomographic image in a pixel of the band tomographic imagecorresponding to the structure of interest and to perform frequencysynthesis on the composite band two-dimensional images to generate thecomposite two-dimensional image.
 12. The image processing deviceaccording to claim 11, wherein the processor is configured to generatethe composite band two-dimensional image that has a pixel value of aband tomographic image determined on the basis of a predeterminedpriority of the structure of interest in a case in which a plurality ofthe band tomographic images are selected in pixels, which correspond toa pixel of the composite band two-dimensional image, in the plurality ofband tomographic images.
 13. The image processing device according toclaim 9, wherein the processor is configured to combine the plurality oftomographic images to generate a first composite two-dimensional image,to generate a composite band two-dimensional image for each frequencyband using the band tomographic image selected for each type of thestructure of interest in a pixel of the band tomographic imagecorresponding to the structure of interest, to perform frequencysynthesis on the composite band two-dimensional images to generate asecond composite two-dimensional image for each type of the structure ofinterest;, and combine the second composite two-dimensional imagegenerated for each type of the structure of interest with the firstcomposite two-dimensional image to generate the compositetwo-dimensional image.
 14. The image processing device according toclaim 13, wherein the processor is configured to replace a pixel valueof the structure of interest in the first composite two-dimensionalimage with a pixel value of the structure of interest in the secondcomposite two-dimensional image to combine the second compositetwo-dimensional image with the first composite two-dimensional image.15. The image processing device according to claim 14, wherein theprocessor is configured to generate the composite two-dimensional imagehaving a pixel value of the second composite two-dimensional imagedetermined on the basis of a predetermined priority of the structure ofinterest in a case in which a plurality of types of the structures ofinterest are included in corresponding pixels of the plurality of secondcomposite two-dimensional images.
 16. The image processing deviceaccording to claim 9, wherein the processor is configured to combine theplurality of tomographic images to generate a first compositetwo-dimensional image; to extract a region of a predetermined specifictype of structure of interest from the first composite two-dimensionalimage; and generate a composite band two-dimensional image for eachfrequency band, using the band tomographic image selected for each oftypes of structures of interest other than the specific type ofstructure of interest, in pixels of the band tomographic image whichcorrespond to the other structures of interest, to perform frequencysynthesis on the composite band two-dimensional images to generate asecond composite two-dimensional image for each type of the otherstructures of interest, to combine the second composite two-dimensionalimages for the other structures of interest with the first compositetwo-dimensional image, and to combine the region of the specific type ofstructure of interest with the first composite two-dimensional image,with which the second composite two-dimensional images have beencombined, to generate the composite two-dimensional image.
 17. The imageprocessing device according to claim 16, wherein the specific structureof interest is a calcification, and the other structures of interest area tumor and a spicula.
 18. The image processing device according toclaim 16, wherein the processor is configured to replace a pixel valueof the structure of interest in the first composite two-dimensionalimage with a pixel value of the structure of interest in the secondcomposite two-dimensional image to combine the second compositetwo-dimensional image with the first composite two-dimensional image.19. The image processing device according to claim 18, wherein theprocessor is configured to generate the composite two-dimensional imagehaving a pixel value of the second composite two-dimensional imagedetermined on the basis of a predetermined priority of the structure ofinterest in a case in which a plurality of types of the other structuresof interest are included in corresponding pixels of the plurality ofsecond composite two-dimensional images.
 20. The image processing deviceaccording to claim 16, wherein the processor is configured to replace apixel value of the structure of interest in the first compositetwo-dimensional image, with which the second composite two-dimensionalimage has been combined, with a pixel value of the region of thespecific type of structure of interest to combine the region of thespecific type of structure of interest with the first compositetwo-dimensional image with which the second composite two-dimensionalimage has been combined.
 21. An image processing method comprising:detecting a structure of interest from a plurality of tomographic imagesindicating a plurality of tomographic planes of an object; selecting atomographic image from the plurality of tomographic images according toa frequency band in a region in which the structure of interest has beendetected; and generating a composite two-dimensional image using theselected tomographic image in the region in which the structure ofinterest has been detected and using a predetermined tomographic imagein a region in which the structure of interest has not been detected.22. A non-transitory computer-readable storage medium that stores animage processing program that causes a computer to execute: a procedureof detecting a structure of interest from a plurality of tomographicimages indicating a plurality of tomographic planes of an object; aprocedure of selecting a tomographic image from the plurality oftomographic images according to a frequency band in a region in whichthe structure of interest has been detected; and a procedure ofgenerating a composite two-dimensional image using the selectedtomographic image in the region in which the structure of interest hasbeen detected and using a predetermined tomographic image in a region inwhich the structure of interest has not been detected.